assert_series_equal considers Nan is not different from real value

[mingi3314]

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Issue description

assert_series_equal considers both Nan and real value as same.

I think this is because testing.asserts._assert_series_inner only check both left and right are null or not.

polars/py-polars/polars/testing/asserts.py

Line 188 in b311405

(left.is_null() != right.is_null()).any()

For example, let left is a series filled with nan values, and right is a series filled with a real values.
Then left.is_null() returns a series filled with False, and right.is_null() returns a series filled with False.
So both series are considered as same when checking missing values.

Suggestion

So, how about change

polars/py-polars/polars/testing/asserts.py

Line 188 in b311405

(left.is_null() != right.is_null()).any()

like as follows?

(left.fill_nan(None).is_null() != right.fill_nan(None).is_null()).any() 

If it's okay, can I work on it and submit a pull request?

Reproducible example

import polars as pl
from polars.testing import assert_series_equal

arr1 = pl.Series("a", [1.0, 2.0, 3.0, 4.0, 5.0])
arr2 = pl.Series("b", [1.0, 2.0, 3.0, 4.0, float("nan")])
arr3 = pl.Series("c", [1.0, 2.0, 3.0, 4.0, None])

assert_series_equal(arr1, arr2, check_names=False) # case 1
assert_series_equal(arr1, arr3, check_names=False) # case 2

Expected behavior

both case 1 and case 2 must raise AssertionError: Series are different
however, only case 2 raise AssertionError, whereas case 1 is considered as equal series

Installed versions

---Version info---
Polars: 0.15.2
Index type: UInt32
Platform: Linux-5.15.74.2-microsoft-standard-WSL2-x86_64-with-glibc2.31
Python: 3.9.12 (main, Apr 26 2022, 13:27:20) 
[GCC 9.4.0]
---Optional dependencies---
pyarrow: <not installed>
pandas: 1.5.2
numpy: 1.22.4
fsspec: <not installed>
connectorx: <not installed>
xlsx2csv: <not installed>
matplotlib: 3.5.2

[alexander-beedie]

@mingi3314; many thanks for the bug report - I found another edge-case while investigating your report, so ended-up extending the assert test coverage and fixing them both 😅